The present invention relates to a print head for a dot matrix printer of an armature attracting type in which an electromagnet attracts an armature to move a print needle for a printing operation, and more particularly, to a structure for regulating the position of the armature at the impact of printing.
In the print head of the attracting type, there are problems in order to increase the printing speed and impact force. One of the problems is that a tip end of the print needle scratches an inked ribbon. In order to prevent the scratching of the inked ribbon, it is necessary to reduce the period of time the print needle contacts the inked ribbon. Namely, if the retraction of the print needle from the ribbon is delayed during high speed printing, the tip end may scratch the ribbon. In particular, when the print gap between the end of the print needle and the printing paper is set at a larger value than a predetermined value for some reason, the scratching of the inked ribbon is liable to occur as describe hereinafter.
Japanese Utility Model Applications Laid-open 61-100536 (hereinafter called first prior art) and 62-27330 (hereinafter called second prior art) disclose print heads each having stopper means.
The print head of the first prior art has a center ring as a stopper. If the print gap is set large, the armature moving to the platen strikes against the center ring, so that the armature is stopped.
In the print head of the second prior art, a nonmagnetic residual sheet is provided on a core of a solenoid corresponding to a plunger secured to the armature. When a solenoid is excited, the plunger is attracted to the core. In the case of large print gap, the plunger strikes against the residual sheet, so that the position of the armature is regulated at the same time as printing.
In the first prior art, when the armature strikes against the center ring, the armature is deflected by the inertia with the center ring or after the print needle impacts the printing paper. As a result, the gap between the core and the plunger is reduced by the magnitude of the deflection of the armature or becomes zero. On the other hand, although the current applied to the solenoid has been cut off, residual magnetism remains in the core. Accordingly, the armature is attracted to the core which makes it difficult to return the armature to a reset position. Consequently, the print needle scratches the inked ribbon. If the gap between the core and the plunger is increased in order to prevent the plunger from staying at the attracted position, the attracting force of the electromagnet is reduces and hence reduced the impact force of the needle.
In the second prior art, when the plunger strikes the residual sheet, the base end portion of the armature floats from the electromagnet because the plunger is adjacent to the pivot. Consequently, the returning motion of the armature is retarded, so that the scratching of the needle occurs.